M&S says

Keep cool and get your full night’s sleep with this cooling duvet cover. From our Comfortably Cool collection, the material is a Tencel® rich blend, which is not only sustainably sourced from the eucalyptus tree for an ethical choice in duvet cover, but also naturally maintains your temperature and draws moisture from your skin for a supremely cool feel. The Tencel® blend also offers luxurious comfort in its fine fibres, aiding you in getting the sleep you need in those hotter months.